Furnace



Oct. 26,y 1937. E. H. BERRYMAN 2,097,093

ELMER h'. BERRVMAN.

A TTORNE Y Oct. 26, 1937,

E'. H. BERRYMAN FURNAGE Filed om. 1a, 1933 2 sheets-sheet 2 INVENTORrELMER H. BERRY/1MM A-TTORNE Y Patented Oct. 26, 1937 UNITED STATESPATENT OFFICE 7 Claims.

This invention relates in general to casting furnaces and in particularto a pneumatically operated ldental casting furnace. f

To make a casting that is exactly complementary to the mold from whichit is made the casting material should of course be maintained in amolten state until it has completely filled the mold. This requires thatthe temperature of the casting material be maintained above its meltingpoint during the entire process and that a su'ostantially constant anduniform pressure be applied to it during the casting operation. Sincemost materials contract upon cooling and solidify- 1 ing it is essentialin making a Vdental casting that the casting material cools andsolidifies progressively from the point furthest away from the spruehole to the entrance of the sprue hole. I have found that a closed airtight muflie forms the most convenient equipment in which a technique ofthis kind can be carried out.

In this method of making castings the investment ring is subjected to amore continuous application of elevated temperatures and consequentlysome means must be resorted to to protect the ring as much as possible.

The length of time required to melt the casting material preparatory tocasting depends not only upon the temperature used but also upon theamount of material and its melting point. The simplest method ofdetermining when the casting material has reached a molten state is tolook at it periodically. The light given off from a molten metal is veryintense and I have therefore found it desirable to view the material ithrough a colored sight glass or window.

In general the object of my invention is the provision of apneumatically operated casting furnace by which the casting material canbe maintained above the melting point during the entire operation andsubjected to a substantially constant and uniform pressure during theactual casting period.

Another object of my invention is the provision of a pneumaticallyoperated casting furnace the muiile of which has a substantiallynegligible volumetric gas capacity in comparison to the gas receiverwhich serves as the source of pressure.

A further object of my inventionV is the provision of a furnace having acolored Window so that the action of the molten metal within the mulemay be watched.

Another object of my invention is the provision of an investment ringhaving a set of bayonet pins at each end so that the ends areinterchange- (Cl. ,Z2-57.1)

able, one set of pins being used to lock the investment ring in placewhiie the other set serves as a means by which the ring may be grippedby a pair of tongs having apertured ends adapted to engage the pins.

Still another object of my invention is the provision of a sprue holderand Crucible former to be used in conjunction with an investment ring toform a protective layer of investment material over that end of the ringwhich is subjected to 10 the most intense heat.

Another object of my invention is the provision intermediate the furnaceand the source of air pressure of a receiver having a volumetriccapacity such that the volumetric capacity of the muiiie 15 isnegligible in comparison to it.

The invention possesses other advantageous features, some of which withthe foregoing, will be set forth at length in the following descriptionwhere I shall outline in full the form of my in- 20 vention which I haveselected for illustration in the drawings accompanying and forming apart of the present specification. In said drawings,

I have shown one form of my invention, but it is to be understood that Ido not limit myself to 25 such form, since the invention as set forth inthe claims, may be embodied in a plurality of forms.

Referring to the drawings:

Figure 1 is a vertical mid-section of the casting furnace taken on thebroken line l--I of Fig- 30" ure 2.

Figure 2 is a horizontal section taken on the line 2-2 of Figure 1.

Figure 3 is a vertical mid-section taken through the investment ring andits associated sprue hold- 35 er and crucible former.

Figure 4 is a diagrammatic representation of the furnace in combinationwith a source of high pressure and an intermediate gas receiver.

Figure 5 is a vertical midsection of a larger investment ring ofmodified construction.

Figure 6 is a section on the line 6-6 of Figure 4.

As best shown in Figure l the furnace comprises a generally cylindricalmetal casing i 45 provided at its base with inwardly and outwardlyextending anges 2 and 3 respectively and bolted to the outwardlyextending flange of a depending generally semi-circular carriage orapron 4. If desired a gasket 4 may be placed 50 between the casing I andthe apron 4 to decrease the transfer of heat between these two elements.Supported on the upper face of the flange 2 is a hollow inverted funnelshaped muiile 5 having an upwardly extending stem 6. Forming part 55 ofthe muifle 5 is an annular plate l the periphery of which forms achannel 8 which accommodates a gasket 9. The peripheral edge of themuffle 5 rests on the gasket 9 and its lower surface overlies an annularrefractory plate lil. The muiiie 5 and its associated plate l are madeof quartz or other refractory material. `Fused to the upper end of thestem 5 is a colored quartz window l l.

Diametrically opposed pockets i2 and i3 formed on the outer surface ofthe casing l accommodate electric contact members lf3 and l5 which arebolted in place as clearly shown in Figure l. The contact members andthe bolts by which they are to be secured to the casing may be keyed toprevent any relative rotation and are insulated from the casing by meansof insulating washers l and mica tubes l5. The free ends of the contactmembers lli and i5 are forked or bifurcated and are formed with curvedsurfaces i'l. As shown in Figure 2 two sets of spaced contact membersare provided.V Spaced electric heating elements i8 and i9 aresymmetrically disposed within the mule 5, one on either side of the muiestem To the ends of these heating elements are sweated Ascaloy caps 29provided with outwardly extending vertical webs 25 and with curvedshoulders 2i complementary to the curved surfaces il of the contactmembers. The curved tines of the contact members straddle the webs 25and are resiliently seated on the complementary curved shoulders 2i.From this construction it will be noted that contact is alwaysmaintained between the contact members and the heating elements over theentire surface of the curved shoulders 2l regardless of the expansion'or contraction of the heating elements due to changes in temperature.

As shown in Figure l the heating'elements i8 and i9 pass throughapertures formed in the casing l and in the mufile 5, and it is to beparticularly noted that the muiiie 5 is only deep enough to accommodatethe heating elements, and therefore has what may be termed a minimum airor gas capacity. This feature is of considerable importance as will beelxplained later.

The top of the casing l is covered by means of a disc 22 supported onthe shoulder '23 formed on the inner` surface of the casing i. The muilestem 6 is sealed to the disc 22 by means of a boss 2li ldepending fromthe plate 22.

The entire cylindrical casing i is enclosed by meansof a bell 25, the.lower peripheral edge of which is supported on a gasket 26, carried in agroove formed in the outwardly extending flange 3 of the casing l.Screws 2l secure the bell 25 to the casing l and hold its peripheraledge sealed against the gasket 26. Gasket washers 28 disposed beneaththe heads of the screws 2 prevent the leakage of air or gas outwardlythrough the bell. The disc 22 and the bell '25 are provided with centralapertures 2S and 3l adapted to register with the mufle stem 5, and'consequently an unobstructed view into the center of the muflle ispermitted. A short stem 32 is provided on the bell 25 and is internallythreaded to accommodate a threaded plug 33. Engaged by the piug etVbetween twoy gaskets 3d and 35 is a second transparent quartz or glasswindow 35 which may be either colored or clear.

As shown in Figure 2 the mufle 5 is of a substantially rectangularshape. The stem 6 how- Y ever is of circular cross-section at its upperend.

but merges with the mule in an ellipse. This construction thereforegives an extended unobstructed view into the mulile between the heatingelements and permits a greater area to be viewed within the muiiie thanwould be permitted by a cylindrical stem.

The casing l is provided with passageways 3l' and 38 which establishcommunication between the interior of the 5 and a gas receiver whichwill be explained later in connection with Figure 4l. The passageways 3land 33 are formed in part (as shown in Figure l) by registeringapertures 35i, (il, ft2, and i3 formed in the plate 'l and in therefractory disc i6.

To the lower end of the carriage or apron 4 is secured a plate. 45, byany suitable means, such as screws It@ and threaded to this plate is ascrew al the lower end of which is provided with a crank LES, while toits upper end is swiveled a plate 5. A cup 5l provided with a iiange 52and formed with counterbores and 53 is secured to the plate i9 by meansof screws 51%. |ihe ange is formed with diametrically opposed notches 55adapted to straddle ribs 55 and 5l formed on the interior surface of theapron 5. It will therefore be seen that since by this construction therotational movement of the cup 5l is prevented, the cup of necessitymust move in a vertical direction rotation of the crank 8. The upperface of the flange 52 is recessed to accommodate a gasket 5S, andalternate rings 5S and 6i are formed on the floor of this recess and onthe lower of the inwardly extending flange 2 for the purpose of bitinginto the gasket 53 to thereby form a tighter seal between the cup 5i andthe flange 2 when the cup is in its uppermost position. If desired thegasket 58 may be held in place by small pins 52 secured to the ange 52,and having their heads bent over into the gasket.

Secured'within the cup 5l is a sleeve provided with diametricallyopposed bayonet slots Gi. Disposed within the cup 5l is an investmentring which is adapted to be locked within the cup by the engagement ofthe bayonet pins S5 within the bayonet slots l. The lower end of theinvestment ring 55 is sealed against an annular gasket 5i' placed on thefloor of the cup 5l and a screen 53 accommodated in the counterbore 53prevents foreign matter such as particles of investment from pluggingthe gas outlet 55 provided in the counterbore 53'. The gas outlet 59communicates through a valve 'il as shown in Figure 4, either with theatmosphere or with a source of vacuum. The contact members id and l5 maybe connected to a source of electric current by leads is provided withtwo sets of bayonet pins, one

set 55, and another set l2 at its opposite end. As has already beenexplained one set is used for locking the ring within the cup 5l. Theother set serves as a means. by which a pair of tongs having aperturedends may be used to lock high pressure tank 85.

the investment ring in place or to dsengage it. This feature is ofimportance, for it has been found that considerable force is sometimesrequired to disengage the investment ring after it has been subjected tothe heat within the muffle, and the application of the ordinary tongs tothe periphery of the ring scores the ring within a short time. By makingboth ends of the ring identical and providing each end with a set ofbayonet pins, either end can be made to serve as the bottom or the topand no damage is done to the ring in handling it.

The upper end of the ring is shown as capped by a sprue holder andcrucible former 13. This member is used only when the mold within theinvestment ring is being formed and serves three purposes. A centralconvex boss 14 forms a concave depression or crucible in the investmentmaterial for the metal button used in making the casting. A small hole15 formed in the boss 14 holds the sprue in place and the sloping walls18 form an annular shoulder of investment material which completelyoverhangs or covers the upper peripheral edge of the investment ring.The purpose of this overhanging shoulder is to protect the upper edge ofthe investment ring against the intense heat to which it would otherwisebe directly exposed when within the muiiie. As shown in Figures l and 4,pipes 8| communicate with the passageways 31 and 38 leading to theinterior of the muiile and are connected by means of a valve 82 with agas receiver 83. The gas receiver in turn communicates through apressure reducing valve 84 with a The volumetric air or gas capacity ofthe mufile should be negligible in comparison with the volumetriccapacity of the gas receiver 83.

To fully appreciate this relationship some consideration should be givento the operation of the furnace. After the mold for the casting has beenprepared in the investment ring, a button of metal placed in thecrucible formed in the upper part of the ring, the investment ringlocked in the cup 5| and the cup 5| locked in its uppermost position soas to form a closed chamber with the muiie 5, the heating elements |8are connected with the source of current. By looking through thewind-ows and 38 which are in axial alignment with the investment ring itcan easily be determined when the metal has been heated to a moltenstate. Previously, the valves 1| and 82 have been closed and thepressure reducing Valve 84 adjusted to maintain an operating pressure ofsay 30 pounds within the gas receiver 83. The pressure within the tank85 may be of course run as high as 1500 pounds. When the metal withinthe muille has been completely melted, valve 82 is opened thereby sub-1, jecting the interior of the muflie and the molten metal tosubstantially the pressure of the gas receiver 83. The molten metal isforced down through the sprue hole into the mold completely lling it anddisplacing the air which was previously contained therein. Thisdisplaced air is driven downwardly through the porous investmentmaterial into the bores 53 and 53 of the cup 5|. The volumetric capacityof the bores 53 and 53' is of course somewhat greater than the capacityof the mold and sprue hole. The

temperature within the munie is maintained throughout the period thatthe casting is being made and consequently the metal within the moldtends to solidify from the bottom upwardly instead of in the reversedirection as would be the case if the heat were discontinued. By havingthe volumetric capacity of the mufile negligible as compared with thecapacity of the gas receiver 83 the molten metal is subjected to asubstantially constant pressure. For let V1 represent the volume of thereceiver 83, P1 its initial pressure, V2 the combined volume of themufi'le and the receiver and Pz the nal equalized pressure within themuflle and the receiver after the valve 82 has been opened. By applyingthe well known law ViXPi and bearing in mind that in the presentinstance V2 is substantially equal to Vi, it will be seen that P2 issubstantially equal to P1. It is therefore desirable to have arelatively large volume of gas available at substantially the operatingpressure and an intermediate receiver answers this requirement mostconveniently. In the absence of the gas receiver 83 the gas pressurewithin the pipes leading from the pressure reducing valve 84 would bematerially reduced when the valve 82 is opened for the small quantity ofgas contained in this pipe would be expanded into the mulfle. Bymaintaining the capacity of the mufile and the counterbores in the cup5| as small as possible and using a relatively large gas receiver thereis substantially no drop in pressure and consequently a denser andbetter defined casting can be made.

The design of the investment ring permits of its being manipulatedwithout injury and the shoulder of investment material formed over itsfree edge protects it from the intense heat to which it would otherwisebe subjected. The use of a, colored window in the mufile stem reducesthe intensity of the light emitted by the molten metal to such an extentthat it can be comfortably watched to determine its condition. Thecomplementary curved contacting surfaces of the heating elements and thecontact members insures a good electrical connection between thesemembers regardless of any changes in the length of the heating elementsdue to temperature variations. Furthermore 'because of its particulardesign the furnace above described can be easily constructed, operated,inspected and repaired. To disassemble the muflie and replace theheating elements it is only necessary to remove the bell 25 byunscrewing the screws 21. The cup 5| and its operating screw are readilyremoved by simply unscrewing the screws 4S and the cup itself can beremoved by means of the screws 54. The casing and the depending carriageor apron 4 have been made as two distinct elements largely to diminishthe transfer of heat from the former to the latter.

For the purpose of making large castings I have shown in Figure 5 alarge investment ring which although of a somewhat diiferentconstruction than the ring above described can be operated in exactlythe same manner. As shown the ring has a cylindrical base 9| and taperedopen neck 92 which merges with a shoulder 93. The diameter of the base9| is such that the base can be supported on the gasket 58 of the cup 5|and the tapered neck 92 is small enough to be accommodated within theopening formed at the center of the inwardly extending ange 2. Anannular gasket 94 is adapted to be disposed between the shoulder 93 andthe lower face of the flange 2 and effects a seal between these twomembers. This sealing action is aided by the beads or rings 5i formed onthe ange 2 and beads or rings formed of the shoulder 93. When thisinvestment ring is sealed in this way between the cup 5| and the ange 2the casting operation can be carried out under pressure in exactly thesame manner as when a small ring is used and with the same advantages.

The screen 66 may be replaced by an inverted cup (not shown) providedwith a perforated bottom and disposed within the cup 5| with its bottom(inverted) slightly below the level of the top of the cup 5l. In effectthis merely raises the level of the screen 68 and decreases thevolumetric capacity of the cup 5I when the large investment ring isbeing used.

I claim:

1. A furnace comprising a casing provided with a depending apron; ashallow inverted funnel shaped mule supported within said casing; aheating element disposed within said muflie at right angles to the axisthereof and a cup slidably associated with said apron and arranged inits uppermost position to close the lower open side of the muille.

2. A furnace comprising a casing provided with a depending generallysemi-cylindrical apron; a shallow inverted funnel shaped mufe supportedwithin said casing; a heating element disposed within said muiile atright angles to the axis thereof and a cup slidably disposed within saidapron and arranged in its uppermost position to close the lower openside of said muiile.

3. A furnace comprising a casing provided with a depending apron; ashallow inverted funnel shaped muie supported within said casing; acounterbored cup slidably associated with said apron and arranged in itsuppermost position to close the lower open side of said muflle; a gasline communicating with the interior of the muiile and a gas outletprovided in the counterbore of the cup. 1

4. A furnace comprising a casing provided with a depending apron and aninwardly extending flange; a muiiie supported on the upper face of saidange and a cup slidably associated with said apron and adapted in itsuppermost position to engage the lower face of said ange and therebyform a closed chamber with said muile.

5. A furnace comprising a casing provided with a depending apron andinternal and external flanges; a muiile supported on the upper face ofsaid internal flange; a cup slidably associated with said apron and-adapted in its uppermost position to engage the lower face of saidinternal flange; and a bell inclosing said cylindrical casing andsupported on its external flange.

6. A furnace comprising a casing having inwardly and outwardly extendingflanges and a depending carriage; a shallow inverted funnel shapedmuffle supported on the upper face of said inwardly extending flange; acounterbored work retaining cup slidably associated with said dependingcarriage and arranged in its uppermost position to close the lower openface of said muile, a cover disposed over the upper end of the casing;diametrically opposed recesses formed on the outer surface of the casingand electric contact members supported therein; electric heatingelements secured to said contact members and extending through thecasing into the muffle; a bell enclosing said casing and supported onsaid outwardly extending flange; a gas inlet associated wtih the muiiieand a gas outlet associated with said work retaining cup. Y

7. A furnace comprising'a casing provided with a depending apron; amufile having an open lower face supported within said casing; a screwassociated with said apron; and a cup swiveled to the upper end of saidscrew and adapted in the extended position of the screw to close theopen face of the muilie.

ELMER H. BERRYMAN.

